This invention relates to enhancing foam generation in an operation for refining molten glass under vacuum. In commonly assigned U.S. patent application Ser. No. 894,143 filed Aug. 7, 1986, there is disclosed a method and apparatus wherein the creation of foam in a vacuum refiner is encouraged by introducing molten glass into the upper headspace of the vacuum chamber. The molten glass immediately foams as it encounters the reduced pressure, and it has been found that foaming the glass is not a problem as had been thought previously, but is highly beneficial in removing dissolved and entrained gases from the melt. Moreover, it has been found advantageous to generate the foam above the liquid level in the refiner vessel, and preferably above the foam layer, so as to subject the foam to the lowest pressure in the vessel. The removal of gases from the liquid phase is greatly enhanced by the large surface area provided to the liquid in the bubble membranes of the foam. Thus, very thin liquid layers are subjected to low pressure in the foam within the vacuum chamber.
Under certain conditions, however, not all of the incoming molten glass stream foams immediately upon entering the vacuum space. In those cases, surface portions of the stream may foam quickly, but a central portion of the stream may penetrate unfoamed into the foam layer or even to the underlying molten pool. If the molten glass does not foam above the foam layer, it may not be exposed to the lowest pressure in the system and thus would be subjected to less than the optimum refining conditions, because even in the relatively light foam layer, pressures increase substantially at lower elevations. Any incoming glass that penetrates to the underlying liquid runs a considerable risk that it will not be exposed to sufficient vacuum to be refined and may be carried into the outgoing stream where it would degrade the quality of otherwise good product. Foaming of the incoming stream of glass may be improved by providing lower pressure in the vacuum chamber and/or reducing the glass flow rate, but it would be desirable to improve the degree to which the stream foams without altering such predetermined parameters as throughput and pressure.
Prior art proposals to refine glass by vacuum have generally avoided foaming the glass rather than enhancing it. Prior to the aforesaid co-pending application, molten glass was not injected into the upper headspace of a vacuum chamber. Therefore, there is no guidance in the prior art as to enhancing foam formation in such an operation.